Real time clock generators have various applications. They can be used for providing timing information for operating systems, for enforcing policies for time-sensitive data, for assisting in positioning calculations and the like. U.S patent application serial number 2002/0083284 of Matsubara et al., titled “Data reproduction system, data recorder and data reader preventing fraudulent usage by monitoring reproducible time limit”, U.S. Pat. No. 5,920,727 of Kikinis et al., titled “Timer-controlled computer system shutdown and startup”, U.S patent application serial number 2004/0128528 of Poisner titled “Trusted real time clock” and U.S patent application serial number 2004/0225439 of Gronemeyer, titled “Method and apparatus for real time clock (RTC) brownout detection, all being incorporated herein by reference, illustrate some usages of real time clocks.
Real time clock signals can be tampered for various reasons including copyrighting piracy, concealing hacking or tampering attempts, reducing the functionality of a device and the like.
Modern integrated circuit providers are required to monitor the real time clocks signal and especially to detect if the frequency of the clock signal is outside a predefined frequency window. Real time clock monitoring circuits, such as sampling circuits or oscillating circuits operate at a higher monitoring frequency than the real time clock frequency. Typically the monitoring frequency is at least twice of the real time clock frequency, such as to comply with the Nyquist sampling rule.
The power consumption of a real time clock monitoring circuit is responsive to the monitoring frequency thus these real time clock monitoring circuits are power consuming. U.S. patent application serial number 2004/0225439 of Gronemeyer describes a constantly operating real time clock signal monitor.
Various apparatuses such as mobile phones, personal data appliances, media players and the like are required to operate for long time periods while using relatively small batteries. Using high power consuming real time clock signal monitoring circuits can reduce the operating period of such apparatuses.
There is a need to provide efficient methods and devices for performing real time clock monitoring.